Observing Asteroid Occultations

Monitoring / recording asteroid occultations of stars is highly valued among planetary observers / scientists. It provides precise position and size measurements of small objects with two time measurements, beginning and duration.

Being in the right place at the right time is an advantage that many amateurs have in their back gardens.

Stars of magnitude 11 and brighter can be monitored visualy for occultation events (see the predictions page). Fainter objects can be followed with increasing apertures. The star/asteroid should be sufficiently bright to be seen clearly in the eyepiece so that a disappearance and reappearance are distinctly detectable. One should test the visibility of the target with the telescope and eyepiece combination to find a usable threshold. In the event that the asteroid and star are of similar brightness (which happens quite often), it is the light drop from the combined magnitude of the star and asteroid, down to that of the asteroid alone that has to be detected. If the magnitude drop is 0.7 or less, then the occultation is unlikely to be seen visually (0.7 is regarded as a cutoff point). Small magnitude drops down to 0.2 can be recorded by video, and under favouable conditions ( good signal to noise) and suitable digital processing down to 0.1.

The occultaion of a bright star by a faint asteroid can result in an apperent total extinction of the star.

Asteroid occultations predictions are based on current knowledge of the asteroid orbit. The tracks are therefore uncertain, and are often shifted by up to a path width, and for smaller objects much further. Occultation are not guaranteed, but improvemts in predictions and star catalogues have improved the situation in the last 10 years. Observers now have the confidence to use moble instruments to improve there chances of success, and to record
a positive.

Some observers use a video camera and TV monitor as an electronic eyepiece with very good results, since the star and asteroid are "amplified" making visual timing with a stopwatch easier. There is still the observer reaction time, and a video delay to take into account when reporting (video delay = half the integration time)

Audio Recording:
The simplest method is to use stopwatches, or an audio tape recorder and ticking clock. (i.e. radio time signal, analogue quartz clock, electronic metronome etc). The aim should be to produce continuous 1 second pulses overlayed with your verbal observation ("In" "Out" etc.). The clock should be synchronised to a primary time source such as radio ( MSF, DCF, WWV) or BT land-line in the UK (dial 123). Remember to announce a minute marker at the beginning and end of the observation so that the seconds can be identified later. Alternatively use a camcorder as an audio tape and use the internal clock which has been set to UT as accurately as possible.

Video and Video Time Stamps
Observers who have sensitive video cameras (WATEC, MINTRON or SUPERCUITS) will obtain the most reliable results when recorded on tape or computer disk. Video also need a time stamp. A GPS time inserter made by Blackbox is available. The CT230 Time and Date Generator supplied by Voltek is another option (not GPS) and is a useful backup. GPS does not work well in a building, best outside with the ariel placed on a metal plate.

The IOTA-VTI is a new addition to video time insetion via GPS, and is designed to give best results for occultations.

Drift Scan
For exclusively asteroid occultations, the CCD drift scan is a good method, but may be specific to the telescope drive software such as WinScan. An 80mm finder scope and DSLR on a good mounting can be used as drift-scan recorder. A stop-watch with a lap mode timer, together with BT telephone time can time the start and end of the drift (instrument not driving) and event time can be deduced to better than 0.5 sec accuracy. Experimentation will be needed to get reliable results. The drift scan mag limit for an 80mm refractor is about 9 using a Canon 20D. Newer cameras have less noisy CCDs and should perform better.

There are other possibilities, but all techniques depend on careful timing. A more detailed overview can be read here Observing Recording and Timing Methods.

Event UT:
The predicted time of an asteroid occultation is reasonably well known. (To within 20 seconds in many cases). It's advisable to begin observing about five minutes before the event. (Having already found the star). Asteroids with well defined orbits can be predicted accurately to within seconds (as can Lunar Occultations). Smaller or more distant objects (e.g. TNOs) with less well defined orbits will have UT errors of up to 1 minute or more. Check the prediction for the event UT. To detect unknown satellites of asteroids, observation will need to commence 10 to 20minutes earlier than the time for the parent asteroid.

Reporting Negative and Positive Asteroid Occultations
All observations are gratefully received. Results will be forwarded to the central body for reducing and archiving. Please contact me for a report form and any questions, as im sure there are many.

Clear Skies and good observing,

Tim

Author: Tim Haymes
Assistant Director (Occultations)
Asteroids and Remote Planets Section
- and Lunar Occultation Coordinator
British Astronomical Association.

Maidenhead,
UK
December 2010